It is known that implanting ions of a rare gas or of hydrogen in a semiconductor material induces the formation of microcavities at a depth proximate to the mean penetration depth of the ions. French Patent Application No. FR-A-2 681 472 discloses a method which uses this property in order to obtain a thin film of semiconductor. This method consists of subjecting a wafer of the desired semiconductor material that includes a flat face, to the following steps a first implantation step by bombarding the flat face of the wafer with ions creating, within the volume of the wafer and at a depth proximate to the penetration depth of the ions, a layer of microcavities separating the wafer into a lower region constituting the mass of the substrate and an upper region constituting the thin film, the ions being chosen from among the ions of rare gases or of hydrogen gas and the temperature of the wafer being maintained below the temperature at which the implanted ions can escape from the semiconductor by diffusion
a second step of bringing the flat face of the wafer into close contact with a support made up of at least one layer of rigid material. This close contact may be created, for example using an adhesive substance, or by the effect of a preliminary preparation of the surfaces and possibly a thermal and/or electrostatic treatment in order to promote interatomic bonding between the support and the wafer;
a third step of thermal treatment of the wafer-support assembly at a temperature greater than the temperature at which the implantation was carried out and sufficient to create, through a crystal rearrangement effect in the wafer and through the pressure of the microcavities, a separation between the thin film and the mass of the substrate. This temperature is, for example 500° C. for silicon.
This implantation is capable of creating a layer of gaseous microbubbles. This layer of microbubbles thus created within the volume of the wafer; at a depth proximate to the mean penetration depth of the ions demarcates, within the volume of the wafer, two regions separated by this layer: one region intended to constitute the thin film and one region forming the rest of the substrate.
According to the implantation conditions, after implantation of a gas, such as, for example hydrogen, cavities or microbubbles may or may not be observable by transmission electronic microscopy. In the case of silicon, it can be obtained microcavities, the size of which can vary from a few nm to several hundreds of nm. Hence, particularly when the implantation temperature is low, these cavities are only observable during the thermal treatment stage, a step during which nucleation is brought about in order to end up with the coalescence of the microcavities at the end of the thermal treatment.
The method described in French Patent Application No. FR-A-2 681 472 does not allow the production of electronic circuits in or at the surface of the flat face of the wafer after the ion implantation step. Indeed, the creation of such circuits implies the carrying out of certain classic micro-electronics operations (diffusion annealing, deposition etc.) that require thermal treatment stages (typically from 40° C. to 700° C.) according to the steps for silicon. At these temperatures, blisters form on the surface of the flat face of the implanted wafer. By way of example, for an implantation of hydrogen ions at a dose of 5.1016 protons/cm2 and at 100 keV energy in a silicon wafer, a thermal treatment carried out at 500° C. for 30 min. leads to degradation of 50% of the surface of the flat face of the wafer, this degradation resulting in the appearance of blisters and to their bursting. It is then no longer possible to properly ensure that the flat face of the wafer is brought into close contact with the support (which will be called the applicator in the subsequent description) so as to detach the semiconductor layer from the rest of the wafer.
This phenomenon of the formation of blisters and craters in the surface of a silicon wafer implanted with hydrogen ions after annealing has been discussed in the article “Investigation of the bubble formation mechanism in a-Si:H films by Fourier-transform infrared microspectroscopy” by Y. Mishima and T. Yagishita, that appeared in the J. Appl. Phys. 64 (8), 15 Oct. 1988, pages 3972-3974.